User`s guide

9
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12 DC Analyses
Overview of Small-Si
g
nal DC
Transfer
The small-signal DC transfer analysis causes the small-signal
transfer function to be calculated by linearizing the circuit
around the bias point. The small-signal gain, input resistance,
and output resistance are calculated and reported.
The digital devices themselves are not included in the small-
signal analysis. A gate, for instance, does not have a frequency
response. Instead, all the digital devices hold the states that were
calculated when solving for the bias point. However, for N and
O devices in the analog/digital interface subcircuits, the analog
side has a well-defined linearized equivalent.
To calculate the small-signal gain, input resistance, and output
resistance, you need to specify an output voltage or current
through a voltage source in the Transfer Function dialog box.
For example, entering V(a,b) as the output variable specifies
that the output variable is the output voltage between two nets,
a and b. Entering I(VDRIV) as the output variable specifies that
the output variable is the current through a voltage source
VDRIV.
You also need to specify the input source name in the Transfer
Function dialog box. The gain from the input source to the
output variable is output along with the input and output
resistances.
For example, if you enter
V(OUT2) as the output variable and V1
as the input source, the input resistance for V1 is calculated, the
output resistance for V(OUT2) is calculated, and the gain from
V1 to V(OUT2) is calculated. All calculations are reported to
the simulation output file.